Contents lists available at ScienceDirect Desalination journal homepage: www.elsevier.com/locate/desal Influence of module orientation and geometry in the membrane distillation of oily seawater Yong Zen Tan a , Le Han a , Wai Hoong Chow a , Anthony G. Fane b , Jia Wei Chew a,b,⁎ a School of Chemical and Biomedical Engineering, Nanyang Technological University, Singapore 637459, Singapore b Singapore Membrane Technology Center, Nanyang Technological University, Singapore 637141, Singapore ARTICLE INFO Keywords: Direct contact membrane distillation (DCMD) Oil emulsion Pore wetting Particulate foulant Module orientation and design Mixed convection flow ABSTRACT To improve the mechanistic understanding for advancing the design and engineering of the membrane dis- tillation (MD) modules, the objective of the current study was to investigate via both experiments and simu- lations the impact of (i) module orientation, (ii) module geometry, and (iii) an oily feed on the permeate flux and pore wetting propensity of direct contact membrane distillation (DCMD). Three module orientations and four feed channel geometries were investigated via experiments and simulations for oily feeds. Two key highlights emanated from this study. Firstly, module orientation mattered for DCMD, particularly in view of the formation of natural thermal convective currents and when the particle density of the particulate foulants varied. Particulate foulants with density much lesser and greater than water only deposited when the membrane was oriented respectively atop and beneath the feed. Secondly, the lack of consideration of convection currents, oil coalescence and the corresponding cake-enhanced temperature polarization in the simulations caused dis- agreement with the experimental results, which underscores the importance of these factors. This highlights that the optimization of MD modules particularly for treating oily feeds requires more mechanistic studies, especially in view of the thermal gradients, rather than relying on analogy with pressure-driven filtration processes. 1. Introduction Membrane distillation (MD) involves distillation through a micro- porous hydrophobic membrane, which acts as a physical interface be- tween the hot feed and cool permeate. It is a promising low-cost, en- ergy-saving alternative (based on use of waste-heat) to conventional separation processes like distillation and reverse osmosis that is gaining much traction, as evidenced in more than ten reviews in the past five years [1–15]. When MD was first described in a patent by Bodell [16] in 1968, it did not immediately become popular for water treatment due in part to the lack of suitable membranes which need to be hydrophobic yet highly permeable to vapors, able to withstand the thermal operating conditions, and are cost-effective [17]. As membrane fabrication ad- vanced, commercial hydrophobic membranes such as polypropylene, PVDF and PTFE used for microfiltration has become viable for MD [17–20], which has improved the potential of MD for treating a mul- titude of feeds. The key advantages of MD include high rejection of solutes, operation at lower pressures because the osmotic pressure difference does not need to be overcome, amenability to make use of waste heat, among others [3]. Despite MD being an attractive green technology, two of the primary issues that plague MD and that are areas of active research are low fluxes and pore-wetting which compromises permeate quality [21]. The treatment of oily wastewater via membrane-based filtration processes has not flourished due to the challenges associated with sustaining the flux and rejection rates [22,23]. However, in view of the large amounts of oily wastewater from the three main contributing industries of oil and gas, palm oil and mining [22–24], improved membrane separation processes could have a role in cost-effectively treating these streams to mitigate the environmental impact associated with their disposal. It is notable that, despite MD being a promising green technology, studies on treating oily feeds via MD are scarce. A recent study on produced water treatment using MD suggested that MD can only be considered for treating low concentrations of oil (500 ppm) and oils with higher proportion of hydrocarbon [25]. Furthermore, another study on shale gas produced water treatment using MD sug- gests that pre-treatment of oil and grease is mandatory prior to MD application to improve stability, quantity and quality of permeate [26]. Yet another study indicated that pore-wetting in MD is not due to the oil itself, but to the interactions between salt, surfactant and the membrane [27]. The focus of this study is on furthering the understanding of MD in treating such oily feeds. http://dx.doi.org/10.1016/j.desal.2017.09.019 Received 21 August 2017; Received in revised form 18 September 2017; Accepted 18 September 2017 ⁎ Corresponding author at: School of Chemical and Biomedical Engineering, Nanyang Technological University, Singapore 637459, Singapore. E-mail address: jchew@ntu.edu.sg (J.W. Chew). Desalination 423 (2017) 111–123 0011-9164/ © 2017 Elsevier B.V. All rights reserved. MARK